1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device, more particularly, a semiconductor device having a gate electrode and a method for manufacturing such a semiconductor device.
2. Description of the Background Art
A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is an exemplary semiconductor device having a gate electrode. Japanese Patent Laying-Open No. 2002-261275 discloses a MOS device having a metal electrode provided on an upper surface of an oxide film provided on an upper surface of 4H type SiC.
A channel mobility is one of performance indexes of a MOSFET. The channel mobility indicates mobility of carriers in a channel. Increase of the channel mobility can lead to reduced on-resistance or increased operation speed. It is known that the channel mobility depends on a plane orientation of the channel surface. Accordingly, a specific plane orientation is employed in a semiconductor substrate used for manufacturing of semiconductor devices. For example, according to the technique described in the above-described patent publication, the plane of the 4H type SiC having the oxide film provided thereon corresponds to the {03-38} plane, or corresponds to a plane having an off angle of 10° or smaller relative to the {03-38} plane.
The patent publication describes a reason why the {03-38} plane allows for a high channel mobility as follows: “Thus, the channel mobility of the MOS device can be increased by providing the oxide film on the surface of SiC having a plane corresponding to the {03-38} plane or a plane having an off angle of 10° or smaller relative to the {03-38} plane. This is presumably due to the following reason. That is, the {0001} plane of SiC is a hexagonal close-packed plane, and therefore has a high density of unbounded chemical bonds per unit area for constituent atoms. Accordingly, interface states are increased to prevent traveling of electrons. In contrast, the {03-38} plane is deviated from the hexagonal close-packed plane, and thus facilitates traveling of electrons. Further, particularly high channel mobility is obtained in the {03-38} plane because even though the {03-38} plane is away from the close-packed plane, bounded chemical bonds of atoms appear relatively cyclically in the surface.”
For example, in the case of using silicon carbide of polytype 4H, it is desirable to employ the {03-38} plane for the channel as described above in order to improve performance of the semiconductor device. Further, according to inspection by the present inventors, it is particularly preferable to employ the (0-33-8) plane of the {03-38} plane. However, the present inventors have found that a normal silicon carbide substrate obtained by cutting an ingot along a plane orientation of (0-33-8) has a surface corresponding to the (0-33-8) plane macroscopically, but has the plane orientation of (0-33-8) at an unexpectedly low ratio microscopically. In other words, it has been found that with the conventional method, the (0-33-8) plane allowing for high channel mobility is not sufficiently effectively employed.
More generally, the present inventors have found that microscopic control for the plane orientation of the channel surface to increase channel mobility has not been examined sufficiently. By increasing the channel mobility, performance of the semiconductor device can be improved. Hence, by microscopically controlling the channel surface, it is expected to obtain a larger channel mobility.